Typically, a cased hydrocarbon well comprises a borehole drilled in a geological formation, a fluid-filled casing disposed in the borehole and cement disposed in an annulus between the casing and the formation.
During well logging operations, it is important to obtain information as to the current condition of the casing. The metallic casing may be exposed to various corrosion factors. For example, corrosion may be due to chemically active corrosive solutions, electrolytic corrosion due to ground currents or contact between dissimilar metals. Further, metallic casing may be subjected to wear, for example, due to abrasion from fluid flowing into the casing. Consequently, over a period of exploitation of a hydrocarbon well, the casing may deteriorate by presenting thin or weakened parts, pits, cracks or holes. Such deteriorations can potentially cause collapse of the casing, leaks of undesired fluid from the geological formation into the casing and in extreme situation loss of the hydrocarbon well. Because the casing is permanently installed in the well borehole, it is nearly impossible to remove the casing for inspection. Thus, various techniques have been developed to inspect the casing in situ to determine the presence and location of deteriorated casing parts.
For this purpose, various methods and apparatuses for characterizing a cased hydrocarbon well are known in the art. For example, the U.S. Pat. No. 6,483,777 describes an apparatus and a method for characterizing a cased well. The apparatus for characterizing a cased well comprises means for insonifying the casing with a pulsed, collimated acoustic excitation aligned at an angle greater than the shear critical angle of the fluid-casing interface, the angle being measured with respect to the normal to the local interior wall of the casing, means for receiving one or more echoes, and means for analyzing the echoes to characterize the cased well.
However, in practice, it has been found that this apparatus is commonly insufficient in order to achieve satisfactory measurements accuracy, in particular due to excessive uncertainty with regards to mud flow velocity into the casing and inner casing dimension (radius or diameter) at determined depths.